The present invention relates to fluid pressure indicating devices and more particularly to a device for detecting and indicating pressure differential between two pressure areas.
By way of example only, the present invention finds particular application in hydraulic systems, where filters are provided to screen out dirt or foreign matter from the hydraulic fluid such as oil passing through the filter. In the course of operation of the hydraulic system, foreign matter accumulates on the filter and the differential oil pressure between the upstream and downstream sides increases, i.e. the upstream side of the filter becomes a high pressure area in relation to the downstream side which becomes a low pressure area. Thus, the magnitude of the pressure differential between the respective areas on either side of the filter represents the extent to which such filter clogging impedes the flow of oil therethrough and thus indicates the operative condition of the filter. Unless such hydraulic filters are timely replaced, i.e. when unduly clogged, the efficiency of the hydraulic system is degraded and continued failure of filter replacement may cause serious impairment of the hydraulic system. It is, therefore, essential that a clear and reliable warning signal be given to the operator when the filter screen becomes unduly clogged so that he may timely replace the clogged filter with a clean one.
In certain applications, such as hydraulic indicator systems in aircraft equipment, it is necessary that such differential pressure indication be visual and independent of the electrical system, giving an unambiguous positive visual indication when in an indicating condition and non-visible when in the non-indicating condition.
It is therefore, an object of the present invention to provide a differential fluid pressure indicator of the character described which is operative independently of an external power source particularly electrical power.
It is another object of the present invention to provide a differential fluid pressure indicator of the character described which provides a discrete visual indication when detecting a differential pressure greater than a preselected level.
It is yet a further object of the present invention to provide a differential fluid pressure indicator of the character described which is provided with means for preventing a false high differential pressure indication which would otherwise be caused by low temperature of the hydraulic fluid.